In a method of cutting metals or non-metals with high precision, a plasma cutter is widely used. An example of the structure of a conventional plasma torch used in a plasma cutter is shown in FIG. 5. A disc or cylindrical material 21 (hereinafter referred to as an electrode material) for resisting wear at high temperature composed of hafnium, tungsten or the like, is buried in the front end of a water cooling type copper-bar electrode 20. In this example of a swirl flow type plasma cutter, the front end of a nozzle 22 is formed in the form of a funnel. This arrangement is a simple means for fixing an arc to the electrode material 21 on the arc axis center in a case where the front end shape of the electrode 20 is flat. A swirl air flow 24 is generated by the funnel-like nozzle 22. An arc column 25 which causes a powerful thermal pinch effect is formed and the directivity of a plasma arc is maintained, thus cutting a workpiece 26. The smaller the nozzle diameter d is, the greater the thermal pinch effect will become. Further, since a pressure several times greater than the atmospheric pressure is distributed inside the arc column 25, this acts to return the pressure to the same pressure as the atmospheric pressure after it is emitted from the nozzle 22. As a result, the arc column 25 has a tendency to be diffused rather than converged.
Regarding a practical machine of a plasma cutter, a voltage of 100 V or thereabouts can be applied in a manual cutting machine and a high voltage of about 200 V can be applied in a high-speed automatic cutting machine. A high arc voltage in such a plasma cutter causes a thermal pinch effect to the arc column 25, whereas, in the nozzle 22, the resistance of a plasma air flow increases because of the smallness of the nozzle opening diameter d, giving rise to a phenomenon that a voltage drop becomes greater. Therefore, if the nozzle opening diameter d of the nozzle 22 is made too small, a voltage drop occurs and the current also decreases, with the result that a plurality of arc columns are generated which reach a workpiece 26 via a section other than the nozzle opening from the electrode 20 in a form in which arc columns coexist with the arc column 25 passing through the nozzle opening of the nozzle 22. FIG. 8 shows the generation of this double-arc phenomenon. In addition, the directivity of the arc column 25 is no longer maintained, resulting in an unstable arc phenomenon such that the arc column 25 swings.
As regards the stable conditions of a plasma arc, an example, in which the ratio of the diameter d of the nozzle opening of the nozzle 22 to the axial length h of the nozzle opening (nozzle constraint ratio h/d) is defined as a general standard value h/d.apprxeq.2, is shown in FIG. 6. It will be understood from this figure that when the nozzle opening diameter d is about 1.5 mm, or smaller, a stable arc generation region (a) almost disappears and becomes either one region of a double-arc generation region (b) or an unstable arc generation region (c). The generation of a plasma arc under these conditions causes a double-arc phenomenon and an unstable arc phenomenon, and therefore the shape of a machined surface of a workpiece and the precision with which the surface of the workpiece is machined deteriorated.
As shown in FIG. 7, regarding the shape of a cut section 27 of a workpiece 26, properly speaking, it must be machined so that D=Da, namely, .THETA.=90.degree. (where D is the machined width in the upper end section of the machined surface 27; Da is the machined width in the lower end section of the machined surface 27; and .THETA. is the angle between the machined surface 27 and its upper end surface). However, because of the above-mentioned double-arc phenomenon or the unstable arc phenomenon, a defective machining such that D&gt;Da or D&lt;Da, or .THETA.&gt;90.degree. or .THETA.&lt;90.degree. is caused. Further, a problem also arises in that the service lifetime of the electrode material 21 decreases due to the swing of the arc column 25 or the like.
A measure has also been proposed in which a magnetic field is made to act on a plasma arc and this is converged (for example, please refer to Japanese Patent Publication No. 62-18978). However, the relationship between the strength of a magnetic field and a nozzle diameter, and machining quality has not been solved, and thus the double-arc phenomenon or the unstable arc phenomenon cannot be prevented and it is therefore difficult to put the above proposed measure into practical use. The present invention has been devised in light of the above-mentioned circumstances. Accordingly, it is an object of the present invention to provide a method of machining plate materials with a plasma cutter which cuts a workpiece using a stable plasma arc and a plasma torch.